1. Field of the Invention
This invention relates to a transport belt and, more particularly, to a transport belt used on machines for the production of web material such as paper or paperboard.
2. Discussion of Background Information
Transport belts are used in the production of paper, for example, in regions in which wet paper material is passed through press sections in order to remove liquid still present in the material. A thin liquid film from the water pressed out of the material forms between the transport belt and the web material as the material passes through two press rollers, for example. Generally, the transport belt is impermeable to water in order to provide as smooth a surface as possible and produce an accordingly unstructured image of this surface on the web material.
However, a problem arises at the point where the transport belt is separated from the web material. More specifically, the existing thin film of liquid or water produces an adhesive effect which impairs the releasing of the web material from the transport belt.
U.S. Pat. No. 6,962,885 B1 attempts to combat this problem by providing, on the side of the transport belt which comes into contact with the web material, a thin layer which does not have a flat or smooth surface structure but a multiplicity of small depressions. The multiplicity of small depressions can be formed during the production of the thin layer by embedding grains of salt in the material. Where the gains of salt are not fully embedded they can be subsequently removed by dissolving in water, thus obtaining hollow spaces or depressions open to the surface. Water pressed out of the web material can be collected in these depressions and the adhesive effect can be at least lessened.
In EP 0 576 115 B1, the surface of the transport belt which comes into contact with the web material is provided with a comparatively rough structure and, this region of the transport belt, is formed from an elastic material. When the material is pressed between two press rollers, the comparatively rough surface is compressed. After the material has passed through this press roller region the compressed transport belt relaxes, with the result that the serrated elevations forming the surface roughness create a disturbance in the existing water film. This makes it easier for the transport belt to separate from the web material.
Another problem with such transport belts is that, in the initial phase of its working life, the surface characteristic of the belt is changed by the contact that takes place with the web material. Particles contained in the web material, for example, micro fibers or the like, are pressed, particularly in press sections, into the surface of the transport belt and remain stuck to or in the region of this surface. Also, the compression of the transport belt's own construction material leads, in the initial phase of the transport belt's working life, to changing surface properties and hence to a changed interaction between the transport belt and the web material. This can impair the releasing of the transport belt from the web material to be produced.